This invention relates to binder compositions having self-hardening properties at room temperature for foundry (or molding) sands for metal-casting molds.
In the foundry industry, binder compositions for producing sand molds are extremely useful and important.
Heretofore, inorganic and organic binders (bonding materials) have been used in sand molds for metal casting. In all of these binders, however, there are advantages and drawbacks, and a fully satisfactory binder has not yet been found.
Representative known methods of hardening foundry sand in which inorganic binders are used are:
1. the method in which water glass is caused to be in a state of adhesion to and on the surfaces of the sand particles, and carbon dioxide gas is blown into the sand thereby to cause it to harden;
2. the method in which water glass and calcium silicate are blended with the sand and caused to harden; and
3. the method in which a cement is blended with the sand, and water is further added to cause a hydration reaction thereby to cause hardening.
All of these methods, however, have been accompanied by the problem of poor breakability of the foundry sand mold, at the time when a casting is being taken out therefrom after the casting has been made by pouring molten metal such as iron or aluminum into the mold, whereby the work efficiency is greatly lowered.
On the other hand, an orgnaic binder, based on a furan resin is known. In the use of this binder, a mixture of a condensation product of furfuryl alcohol-formaldehyde and a condensation product of ureaformaldehyde is caused to harden in the presence of a strong acid such as para-toluenesulfonic acid or phosphoric acid. The hardening speed of the resin is easily affected by the ambient temperature, and the hardening is especially slow in winter. Furthermore, since a strong acid is used, defective hardening tends to occur depending on the kind of sand, as for example, in the case where seashores and is used.
In addition, a binder comprising a mixture of a polyol compound having two or more hydroxyl groups and a polyisocyanate compound having two or more isocyanate groups is known. Binders of this group are disadvantageous in that it is difficult to obtain strong binders because of the poor compatibility or solubility, in general, between the polyol compounds and the polyisocyanate compounds.
Accordingly, in order to improve the compatibility between a polyisocyanate compound and a polyol compound thereby to produce a strong binder, the combined use of a polyol compound containing tertiary nitrogen atom has been proposed. The use of such a compound, however, gives rise in some cases to problems such as casting defects, including blow holes due to the increased content of nitrogen in the binder and a shortening of the work time of the binder mixture due to the catalytic action of the tertiary nitrogen atom.